1. Field of the Invention
The present invention generally relates to wavelength-selectable lasers, and more particularly to a wavelength-selectable laser whose oscillation frequency can be tuned over a required frequency bandwidth.
2. Description of the Related Art
Lasers are composed of an optical cavity including a gain medium, and obtain oscillation by increasing the gain. With the recent rapid increase in demand for communication, wavelength division multiplexing (WDM) communication systems that multiplex a plurality of signal beams having different wavelengths to enable mass transmission through a single optical fiber have been developed. Such wavelength division multiplexing optical fiber communication systems require a wavelength-selectable laser that can select light of a desired wavelength at high speed with accuracy from within a wide wavelength range.
In devices that multiplex wavelengths while controlling oscillation to a desired single wavelength, it is extremely desirable that a wavelength range broader than 60 nm, which corresponds to the two most commonly used frequency bands (1530-1560 nm and 1570-1610 nm) in optical fiber communication devices, be effectively selectable.
Oscillation wavelengths are required to be tuned at a speed faster than a few milliseconds to maintain virtually uninterrupted operation of such devices.
Such tuning is enabled by wavelength-selectable lasers disclosed in U.S. Pat. No. 6,091,744 to Sorin et al. (hereinafter referred to as first prior art), part of U.S. Pat. No. 5,970,076 to Hamada (hereinafter referred to as second prior art), and Japanese Laid-Open Patent Application No. 2000-261086 by Inoue (hereinafter referred to as third prior art).
FIG. 1 is a block diagram showing the configuration of a wavelength-selectable laser according to the first prior art. The wavelength-selectable laser of FIG. 1 includes a gain medium 101, a reflecting mirror 102, a bandpass filter 103, a frequency controller 104, a plurality of fiber Bragg grating (FBG) reflective filters 106, and a single-mode optical fiber 107.
In this configuration, the transmission characteristics of the bandpass filter 103 and the FBGs 106 are tunable. Further, laser oscillation occurs at the frequencies where the peak of bandpass transmission coincides with the peak of FBG reflection. Broad mode tuning is realized by using the FBGs 106 reflecting different narrow frequency bands. The bandpass filter 103 is used to transmit a selected one of the reflection spectra of the FBGs 106. Each of the FBGs 106 is tunable to a desired wavelength within its tuning range which wavelength is connectable to a propagation channel.
When employed in the case of using a large number of FBGs as tunable reflective filters, the above-described method has the disadvantage that two complicated and expensive filters should be tuned simultaneously. With the FBG characteristics being fixed, the above-described method requires as many FBGs as the number of desired wavelength channels, so that the optical cavity is elongated to increase the size and cost of the laser.
Next, FIG. 2 is a block diagram showing a wavelength-selectable laser according to the second prior art. The wavelength-selectable laser of FIG. 2 includes a gain medium 111, a reflecting mirror 112, a collimator lens 113, a reflecting grating 116 formed of a diffraction grating, and a cavity 117.
According to this configuration, wavelength selection is achieved by mechanically rotating the reflecting grating 116. That is, the reflection peak wavelength of the reflecting grating 116 is tuned by rotating the reflecting grating 116.
This configuration has the disadvantage of requiring a large mechanical configuration in size for wavelength tuning. That is, in order to realize the second prior art, the feedback system of the rotation angle of the reflecting grating and a complicated and expensive tuning device are required. Further, mechanical tuning results in a tuning delay of the order of milliseconds with mechanical stability and reliability being a matter of concern.
A wavelength-selectable laser according to the third prior art includes an acousto-optical tunable filter (AOTF) and a gain medium. According to the configuration of the third prior art, part of light generated in the gain medium to have a wide wavelength range is selected by the AOTF. That is, a surface acoustic wave (SAW) is generated by applying an RF signal to the inter digital transducer of the AOTF so that the surface acoustic wave and propagating light interact with each other to switch the polarization mode of propagating light of a specific wavelength corresponding to the frequency of the RF signal between TE and TM. The light of the specific wavelength is selected using a polarization beam splitter.
However, according to the third prior art, laser oscillation is destabilized by the occurrence of a Doppler frequency shift, and it is extremely difficult to select light of a specific single wavelength using the AOTF.
Other devices such as fiber ring lasers including a tunable filter and an erbium-doped fiber amplifier are slow in gain response and require a relatively long period of time of the order of a few milliseconds in switching wavelengths.
In this current situation, stability is required in a single-mode laser device with highly suppressed multi-mode oscillation. In the WDM devices (the optical fiber communication devices), a tuning range of 30 to 60 nm and an oscillation wavelength accuracy of the order of tens of nanometers, for instance, are required.
A signal laser oscillation wavelength should be selected with accuracy in the order of tens of microseconds from a large number of specific wavelengths in a wide wavelength range. Further, in consideration of cost, the control mechanism should be simple with as small a feedback configuration for frequency control as possible.
Thus, there is difficulty in downsizing the conventional wavelength-selectable laser, and it is difficult for the conventional wavelength-selectable laser to select light of a specific wavelength or frequency from a broad frequency range in a short period of time with accuracy.